This invention relates to radioactivity well logging and more particularly to techniques for natural gamma ray logging to determine the characteristics of shale formations.
In the course of interpreting information obtained by modern exploration techniques, it has been found that it is highly desirable to be able to determine the location of so-called "source rocks" for oil production in regions being explored. In the past, magnetic and seismic surveys have been utilized to locate sedimentary basins which contain thick deposits of ocean bottom sediments from ancient times. The seismic or aeromagnetic surveys can locate the general area of petroleum traps or impermeable rock structures in such sedimentary basins prior to the drilling of exploratory wells. It has been also common practice in the past to evaluate rock samples from exploratory wells drilled into suspected petroleum traps located in such sedimentary basins and to evaluate the oil bearing content of the rock from the rock cuttings.
Of course, the foregoing procedures have been successful in some instances and unsuccessful in other instances. The process of gathering core rock samples from an exploratory well for laboratory analysis is very time consuming and expensive because the drilling operation in such a well must be conducted in a careful manner in order to preserve the integrity of the rock cores. Also, core taking bits and attendant careful handling of the cores is required to preserve the information contained in the core samples.
According to the principles of the present invention a much faster method for analyzing the sedimentary rock layers penetrated by an exploratory borehole is provided. It has been found that a correlation may be established between the uranium content or the uranium, potassium and thorium content of shales which can be used as an indicator of the organic matter content of the shales. Thus, according to the techniques of the present invention, when an exploratory borehole is drilled, a natural gamma ray log may be analyzed in terms of the spectral energy distribution of the naturally occurring gamma rays from the earth formations penetrated by the borehole. This information can be interpreted in terms of the uranium, potassium and thorium content of the shales penetrated by the borehole. This radioactive element content of the shales is indicative of its characteristics as a source rock for the production of petroleum. Thus valuable information may be gained using the techniques of the present invention which can result in considerable savings of time and expenditures of money in evaluating the source rock potential of earth formations penetrated by a well borehole.
The invention is pointed out with more particularity in the appended claims. The invention is best understood by reference to the following detailed description thereof when taken in conjunction with the accompanying drawings in which: